This invention relates to Petri dishes.
Petri dishes are well known and in common usage, particularly in laboratories for producing cultures in a culture medium such as agar. Such dishes have a flat bottom portion and short cylindrical sides. Flatness of the bottom portion is important, so that a uniform layer of the culture medium can be produced using as little of the medium as possible.
The market for Petri dishes is highly competitive and price sensitive. It is therefore encumbent on manufacturers to produce Petri dishes at as low a price as possible. At the same time, however, it is of course essential that the quality of the product be excellent.
In Petri dishes, curvature of the bottom is a problem because the bottom tends to develop significant curvature after removal from the mold, especially due to rapid cooling caused by the relatively fast cycle times in production. Curvature is unacceptable because it confuses accurate assessment of the culture in the dish. This problem traditionally has been dealt with by thickening the walls of the dish and increasing the cooling time before it is ejected from the mold. However, these solutions are unsatisfactory because they involve the use of more material than strictly required for the mechanical strength of the finished article, and they lengthen the mold cycle time, thereby making the process expensive while lowering the production rate of the mold.
In general, there is a need for an improved Petri dish, such that manufacturing costs can be reduced and quality either enhanced or at the very least not sacrificed.
Since the cost of materials, i.e. resins, is the primary cost factor in producing Petri dishes, the best way to reduce cost is to reduce the amount of material required. However, it has not hitherto been possible to reduce the amount of material to the extent possible in the present invention, without sacrificing quality.
"Quality" in this context means primarily that the Petri dish must be reasonably strong, and that the bottom must be as close as possible to being absolutely flat. Other factors include transparency and the ability of agar to bond to the dish.